Conventionally, a thermal flow measuring device is employed for measuring intake airflow of an internal combustion engine of a vehicle. A thermal flow measuring device includes a heating resistor and measures airflow in accordance with heat radiation from the heating resistor to the airflow. The heating resistor is constructed by winding a metallic resistive element around a bobbin in a cylindrical shape, and connecting both ends of the resistive element to a pair of lead members. The metallic resistive element is, for example, a platinum thin wire having a large temperature coefficient of resistance. The heating element is coated with a protective film at the surface and an end portion.
Heat dissipated from the heating resistor to airflow is substantially in proportion to the square root of velocity of the airflow and a temperature difference between the surface of the heating resistor and the airflow. The heating resistor and temperature compensating resistive element are combined to construct a resistance bridged circuit. The temperature compensating resistive element is configured to detect temperature of the airflow. As the heating resistor dissipates heat, resistance of the heating resistor changes. An electric current is supplied to the resistance bridged circuit, and the electric current is controlled such that temperature difference between the heating resistor and the temperature compensating resistive element is regularly maintained at a predetermined value. The velocity of the airflow is detected based on the electric current, and the amount of intake air, i.e., air mass flow is measured based on the flow velocity.
In the present structure, when a balanced condition of the resistance bridged circuit changes due to variation in temperature, the change in the balanced conditions can be compensated by the temperature compensating resistive element. However, suspended particulates such as dust are contained in intake air of the internal combustion engine, and such suspended particulates may adhere to the surface of the heating resistor and the lead members. Such adhesion changes a heat radiation characteristic of the heating resistor and a thermal property of the lead member. As a result, an output characteristic such as response of the thermal flow measuring device may be impaired.
In general, the heating resistor is maintained at high temperature, for example, 200° C. Therefore, even when water, which is contained in suspended particulates, adheres to the heating resistor, the water may be evaporated. However, oil contained in suspended particulates may adhere to the heating resistor and remain, even low-boiling-point components of the oil is partially evaporated on the heating resistor. Thus, the surface of the heating resistor may be contaminated by suspended particulates containing oil.
A conventional airflow measuring device has a structure configured to protect the lead member, the heating resistor, and a support member, which supports the heating resistor, from adhesion of suspended particulate as contamination substance contained in airflow.
JP-A-59-190623 proposes a thermal airflow measuring device including heating resistive elements for measuring air mass flow and temperature compensation, and the heating resistive elements are located along airflow in a bypass passage. The bypass passage leads part of airflow from a main passage. The heating resistive elements are inclined with respect to the airflow at an angle preferably less than 90°. In the present structure, the heating resistor for measuring airflow can be elongated with respect to the direction of the airflow in the bypass passage, which is limited in diameter, and whereby enhanced in response.
In the present structure, the heating resistive element and the lead member are inclined against the airflow in some degree, so that part of laminar flow in the airflow is easily shifted to turbulent flow on the surface of the heating resistor. Therefore, a heat transfer characteristic as a heat transfer coefficient is enhanced, so that output response may be improved. However, the airflow is added with a vertical component directed to the surface of the heating resistor, and consequently stagnation is apt to arise in the airflow. Thus, suspended particulates are apt to adhere to the heating resistor, the lead members, and support members that are located perpendicularly to the flow direction.
JP-A-59-190624 proposes a thermal airflow measuring device including a protection member provided upstream of the heating resistor for reducing adhesion of suspended particulates. In the present structure, adhesion of suspended particulates can be suppressed, so that the output characteristic can be maintained.
However, airflow may become unstable due to the protection member, and consequently airflow cannot be accurately measured.
JP-A-8-105778 proposes a thermal airflow measuring device including a heating resistor, which has a relatively large temperature coefficient of resistance. The heating resistor is located in a fluid passage and connected to a bridged circuit. The heating resistor is formed with a cylindrical bobbin having a center axis, which is perpendicular to the airflow. A pair of lead members connects both ends of the heating resistor with a pair of terminals, which is erected in the passage. Each lead member protrudes from each end surface of the bobbin to be in parallel with the center axis of the bobbin. A protection member is provided upstream of each lead member with respect to streamlines of the fluid. The protection member is located in parallel with the lead member so as not to disturb the fluid flow.
The size of the protection member is determined correspondingly to the size of the lead member, whereby adhesion of suspended particulates to the lead member can be suppressed. Consequently, measurement accuracy and a response characteristic can be maintained.
In the present structure, contamination of the surface of the lead member can be suppressed. However, contamination of the heating resistor as an essential component may not be effectively suppressed. The particulates adhered to the surface of the heating resistor may be partially burned or evaporated by heat of the heating resistor. However, suspended particulates, which contain oil, may not be removed and accumulate on the heating resistor.
In reality, intake air contains various kinds of suspended particulates such as very minute particulates, which are significantly light and easily scattered, relatively large and heavy particulates, which are applied with inertia and apt to collide against the surface, moisture, and oil. Such suspended particulates as contamination substance may exist in stagnation and adhere to the heating resistor, the lead member of the heating resistor, and the support member. As a result, the heat radiation characteristic changes, and the output characteristic such as response is impaired.
The above conventional structures are not sufficient to reduce adhesion. Therefore, accuracy in output characteristic such as response of the detection of the airflow may be impaired.